1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to a method for fabricating high-quality semiconductor light-emitting devices on silicon substrates.
2. Related Art
Solid-state light-emitting devices are expected to be the illumination wave of the future. High-brightness light-emitting diodes (HB-LEDs) are emerging in an increasing number of applications, from light source for display devices to light-bulb replacement for conventional lighting. Meanwhile, solid-state lasers continue to beam as the driving force in many critical technological fields, from optical data storage, to optical communication networks, and to medical applications.
In recent years, an increasing demand has emerged for blue light-emitting devices, which include both blue LEDs and blue lasers. These blue light-emitting devices are generally based on wide band-gap semiconductor materials, such as the nitride-based InxGayAl1-x-yN (0<=x<=1, 0<=y<=1) materials and zinc oxide-based ZnxMgyCd1-x-yO (0<=x<=1, 0<=y<=1) or ZnxBeyCd1-x-yO (0≦x≦1, 0≦y≦1) materials, which both are under intense development worldwide. In particular, recent success in the development of nitride-based LEDs and lasers (e.g., GaN-based LEDs and lasers) not only extends the light-emission spectrum to the green, blue, and ultraviolet region, but also can achieve high light emission efficiency.
Successful epitaxial growth of nitride-based materials typically requires matching of the lattice constant and thermal-expansion coefficients of the substrate and epitaxial layers. Consequently, unconventional substrate materials, such as sapphire (Al2O3), gallium arsenide (GaAs), gallium phosphide (GaP), and silicon carbide (SiC), are often used to grow InGaAlN, ZnMgCdO, and ZnBeCdO materials in order to achieve such matching.
However, these unconventional substrates are typically expensive and not available in large diameters. For example, the high costs make SiC substrates unsuitable for large-volume commercial production. Another problem associated with these substrates, with Sapphire substrate in particular, is that they have low electrical and thermal conductivity. As a result, a light-emitting device fabricated on such substrates often requires both positive and negative electrodes to be on the same side of the substrate. However, this lateral-electrode configuration can reduce light-emitting efficiency, increase fabrication complexity, and limit heat dissipation during operation. Hence, it is desirable to find a substrate material which is of low cost, is highly conductive, and facilitates easy fabrication.
Although being the most mature and widely used semiconductor material in semiconductor industry, Silicon (Si) has an indirect energy bandgap and is therefore considered to be unsuitable if used directly as light-emitting materials. Hence, Si has seen very limited use in light-emitting applications in the past. Nevertheless, many research efforts have been attempted to integrate Si with light-emitting devices. Recent successes from these efforts have allowed semiconductor light-emitting materials to be fabricated on conventional Si substrate.
The latest research efforts have been focusing on using Si substrates to manufacture nitride-based light-emitting devices. As a substrate material, silicon has both good electrical and thermal conductivity. Furthermore, the costs of silicon substrates are significantly lower than the costs of sapphire or SiC substrates. It also enables integration of light-emitting devices with Si-based electronics.
Unfortunately, using Si substrate to fabricate InGaAlN and ZnMgCdO based devices faces a serious problem. When exposed to reactant gases containing group-V or group-VI elements (e.g., gases containing N or O, which are typically used in metal organic chemical vapor deposition, MOCVD), Si atoms on the substrate surface can easily react with these elements. The reactions result in an amorphous overcoat formed on top of the substrate surface, which can degrade the quality for subsequent film growth. For example, an amorphous SiNx overcoat tends to form prior to the formation of InxGayAl1-x-yN based layers, and an amorphous SiO2 overcoat tends to form prior to the formation of ZnxMgyCd1-x-yO or ZnxBeyCd1-x-yO based layers. Furthermore, this amorphous overcoat is difficult to remove, and therefore is detrimental to the subsequent fabrication of the light-emitting structure.
To solve the above-described problem, one can deposit an Al transition layer on the Si substrate prior to the growth of InxGayAl1-x-yN, ZnxMgyCd1-x-yO, or ZnxBeyCd1-x-yO based material. The Al transition layer can prevent the oxidation or the nitridation of the Si substrate. However, Al is known to be chemically active. This instability can complicate the overall fabrication process and cause potential issues on device reliability.
Hence, what is needed is a method for preparing Si substrates for fabricating high-quality semiconductor light-emitting structures without above-described problems.